What's a Quantum Computer?
It's a computer that makes use of the quantum states of subatomic particles to store information. While a normal Turing computer can only perform one calculation at a time, a quantum Turing machine can perform many calculations at once.
The following quote is from http://computer.howstuffworks.com/quantum-computer1.htm If you want to get into the nitty-gritty of quantum computing, it's a good article.
It explains the advantage of quantum computing like so:
Today's computers, like a Turing machine, work by manipulating bits that exist in one of two states: a 0 or a 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits, which can exist in superposition. Qubits represent atoms, ions, photons or electrons and their respective control devices that are working together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers.
So What's the Problem?
Well normally we feel that faster is better. But there's one group of people who are worried about quantum speed computing. Crytographers! Quantum computing will be their worst nightmare.
Their concern is if computing gets that fast, it could break RSA encryption which is used to make sure data transmitted on the internet is secure.
How Does that Impact Cryptocurrencies?
Well, grasshopper, quantum computers could also break any digital signatures used in Bitcoin and other cryptocurrencies. That would scare a lot of people.
How Close Are We to Quantum Computing?
We're years away. The building block of this type of computing is the qubit. The article explains how qubits would work in quantum computing. If you're at all geeky, it's pretty interesting. The limitation is that right now qubits are by their nature unstable. So one goal is to reduce error rates.
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